Parasitism and Disease 



3 



familiar examples. These and a host of others 

 all show a very high degree of specialization fit- 

 ting them for their peculiar lives in their hosts. 



From among the insects may be selected interest- 

 ing examples of almost all kinds and degrees of 

 parasitism, temporary, permanent, external, in- 

 ternal (Figs. 4, 5, 6). Among them is found, too, 

 that curious condition known as hyperparasitism 

 where one animal, itself a parasite, is preyed upon 

 by a still smaller parasite. 



"The larger fleas have smaller fleas 

 Upon their backs to bite urn, 

 These little fleas still smaller fleas 

 And so ad infinitum." 



Coming now to the minute, microscopic, one- 

 celled animals, the Protozoa, we find entire 

 groups of them that are living parasitic lives, de- 

 pending wholly on one or more hosts for their ex- 

 istence. Many of these have a very remarkable 

 life-history, living part of the time in one host, 

 part in another. The malarial parasite and others 

 that cause some of the diseases of man and do- 

 mestic animals are among the most important of 

 these. 



PARASITISM 



Among all these parasites, from the highest to 

 the lowest the process that has fitted them for a 



